Recently, a recording medium capable of recording a three-dimensional interference pattern as a spatial change of the refractive index in the inside thereof receives attention. For example, a photorefractive crystal receives attention, which is a photosensitive medium such as lithium niobate (NbO3, abbreviated to LN). The photorefractive effect is a phenomenon in which a movement of electric charge generated by photoexcitation in the medium forms a spatial electric field, and then, the spatial electric field produces linear electrooptic effect, i.e. Pockels effect, and then, this effect produces a change of a refractive index of a medium.
In ferroelectric crystal and the like showing the photorefractive effect, the change of the refractive index responds to a fine light input pattern typically more than 1000 lines per 1 mm, and the change of the refractive index is produced in real time with a response speed of micro second order to second order in some materials. For this reason, various applications are studied with respect to the use of the ferroelectric crystal and the like as a real-time hologram recording medium capable of eliminating the necessity of a development process.
One of the various applications is a hologram memory. This hologram memory has the following features. Digital data is converted into a two-dimensional light-and-dark dot pattern image on a panel plane of a transmission type liquid crystal device for example, by using a spatial light on-off signal. Then, interference between a signal light passing through this image data and a coherent reference light is brought about. Then, the interference pattern corresponding to the dot pattern image is recorded in the recording medium. On the other hand, in reproduction, the recording medium in which the interference pattern is recorded is irradiated with a light identical with the reference light. Then, the resulting dot pattern image is received by a photodetector. Then, processing is performed on an output signal of the photodetector by using an electric circuit in order to reproduce the digital data. Then, the reproduced digital signal is read. For example, Japanese Patent Application Laid-Open No. Hei 11-282330 discloses an example of a hologram memory technology.